Metal porous bodies having a three-dimensional network structure have been used in a wide range of applications, such as various filters, catalyst supports and battery electrodes. For example, Celmet (manufactured by Sumitomo Electric Industries, Ltd., registered trademark) composed of three-dimensional network nickel porous body (hereinafter, referred to as a “nickel porous body”) has been used as an electrode material for batteries, such as nickel-metal hydride batteries and nickel-cadmium batteries. Celmet is a metal porous body having continuous pores and characteristically has a higher porosity (90% or more) than other porous bodies such as metallic nonwoven fabrics. Celmet can be obtained by forming a nickel layer on the surface of the skeleton of a porous resin molded body having continuous pores such as urethane foam, then decomposing the resin molded body by heat treatment, and reducing the nickel. The nickel layer is formed by performing a conductive treatment of applying a carbon powder or the like to the surface of the skeleton of the resin molded body and then depositing nickel by electroplating.
On the other hand, as with nickel, aluminum also has excellent characteristics such as a conductive property, corrosion resistance and lightweight, and for applications in batteries, for example, an aluminum foil in which an active material, such as lithium cobalt oxide, is applied onto the surface thereof has been used as a positive electrode of a lithium battery. In order to increase the capacity of a positive electrode, it is considered that a three-dimensional network aluminum porous body (hereinafter, referred to as an “aluminum porous body”) in which the surface area of aluminum is increased is used and the inside of the aluminum is also filled with an active material. The reason for this is that this allows the active material to be utilized even in an electrode having a large thickness and improves the active material availability ratio per unit area.
As a method for producing an aluminum porous body, Patent Literature 1 describes a method of subjecting a three-dimensional network plastic substrate having an inner continuous space to an aluminum deposition treatment by an arc ion plating method to form a metallic aluminum layer having a thickness of 2 to 20 μm.
It is said that in accordance with this method, an aluminum porous body having a thickness of 2 to 20 μm is obtained, but since this method is based on a vapor deposition method, it is difficult to produce a large-area porous body, and it is difficult to form a layer which is internally uniform depend on the thickness or porosity of the substrate. Further, this method has problems that a formation rate of the aluminum layer is low and production cost is high since equipment for production is expensive. Moreover, when a thick film is formed, there is a possibility that cracks may be produced in the film or aluminum may exfoliate.
Patent Literature 2 describes a method of obtaining an aluminum porous body, including forming a film made of a metal (such as copper) on the skeleton of a resin foam molded body having a three-dimensional network structure, the metal having an ability to form a eutectic alloy at temperatures equal to or lower than the melting point of aluminum, then applying an aluminum paste to the film, and performing a heat treatment in a non-oxidizing atmosphere at a temperature of 550° C. or higher and 750° C. or lower to remove an organic constituent (resin foam) and sinter an aluminum powder.
However, in accordance with this method, a layer which forms a eutectic alloy of the above-mentioned metal and aluminum is produced and an aluminum layer of high purity cannot be formed.
As other methods, it is considered that a resin molded body having a three-dimensional network structure is subjected to aluminum plating. An electroplating method of aluminum itself is known, but since aluminum has high affinity to oxygen and a lower electric potential than hydrogen, the electroplating in a plating bath containing an aqueous solution system is difficult. Thus, conventionally, aluminum electroplating has been studied in a plating bath containing a nonaqueous solution system. For example, as a technique for plating a metal surface with aluminum for the purpose of antioxidation of the metal surface, Patent Literature 3 discloses an aluminum electroplating method in which a low melting composition, which is a blend melt of an onium halide and an aluminum halide, is used as a plating bath, and aluminum is deposited on a cathode while the water content of the plating bath is maintained at 2 mass % or less.
However, in the aluminum electroplating, plating of only a metal surface is possible, and there is no known method of electroplating on the surface of a resin molded body, in particular electroplating on the surface of a resin molded body having a three-dimensional network structure.
The present inventors have made earnest investigations concerning a method of electroplating the surface of a polyurethane resin molded body having a three-dimensional network structure with aluminum, and have found that it is possible to plate a polyurethane resin molded body, in which at least the surface is made electrically conductive, by plating the resin molded body with aluminum in a molten salt bath. These findings have led to completion of a method for producing an aluminum porous body. In accordance with this production method, an aluminum structure having a polyurethane resin molded body as the core of its skeleton is obtained. For some applications such as various filters and catalyst supports, the aluminum structure may be used as a resin-metal composite as it is, but when the aluminum structure is used as a metal structure without resin because of constraints resulting from the usage environment, an aluminum porous body needs to be formed by removing the resin.
Removal of the resin can be performed by any method, including decomposition (dissolution) with an organic solvent, a molten salt or supercritical water, decomposition by heating or the like.
Here, a method of decomposition by heating at high temperatures or the like is convenient, but it involves oxidation of aluminum. Since aluminum is difficult to reduce after being oxidized once as distinct from nickel, if being used in, for example, an electrode material of a battery or the like, the electrode loses a conductive property due to oxidation, and therefore aluminum cannot be used as the electrode material. Thus, the present inventors have completed a method for producing an aluminum porous body, in which an aluminum structure obtained by forming an aluminum layer on the surface of a resin molded body is heated to temperatures equal to or lower than the melting point of aluminum in a state of being immersed in a molten salt while applying a negative potential to the aluminum layer to remove the resin molded body through thermal decomposition to obtain an aluminum porous body, as a method of removing a resin without causing the oxidation of aluminum.
Incidentally, in order to use the aluminum porous body thus obtained as an electrode, it is necessary to attach a lead wire to the aluminum porous body to form a current collector, fill the aluminum porous body serving as the current collector with an active material, and subject the resulting aluminum porous body to treatments such as compressing and cutting by a process shown in FIG. 1, but a technology for practical use for industrially producing electrodes for nonaqueous electrolyte batteries, and capacitors and lithium-ion capacitors, and the like from an aluminum porous body has not yet been known.